The present invention relates to an air-fuel ratio variation suppressing apparatus for an internal combustion engine equipped with an evaporation fuel processing mechanism. This apparatus supplies fuel vapor in a fuel tank to a canister via a vapor passage and purges fuel in the canister to an air-intake passage of the internal combustion engine via a purge passage provided with a purge control valve when the internal combustion engine is operated. This apparatus also supplies the fuel vapor provided in the fuel tank during refueling to the canister via a breather passage and a pressure sensitive valve, which opens in response to the pressure in the fuel tank during refueling.
Evaporation fuel processing apparatuses have conventionally been used to prevent fuel vapor generated in the fuel tank of a vehicle from leaking into the air (see Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-144870 and Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-189426, for example).
Those evaporation fuel processing apparatuses have a vapor passage that connects the interior of the fuel tank to the interior of the canister, and the fuel vapor generated in the fuel tank is led into the canister via the vapor passage. The fuel vapor is discharged into the atmosphere via an atmosphere release passage after most of its fuel component is captured by an adsorbent, like activated carbon, in the canister. The fuel component adsorbed by the adsorbent is separated from the adsorbent by air that is newly supplied into the canister from outside via an atmosphere-intake passage and is then supplied to the air-intake passage of the engine to be burned together with the intake fuel.
In an internal combustion engine equipped with such an evaporation fuel processing apparatus, the fuel that is purged in the air-intake passage affects air-fuel ratio control. Particularly when the concentration of the fuel to be purged is rich, the effect of purging on the air-fuel ratio control is noticeable. In this respect, the scheme disclosed in the aforementioned Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-144870 determines the state of the fuel vapor produced in the fuel tank based on the purge progressing time and the amount of evaporation fuel generated in the fuel tank. The apparatus of this publication restricts the purge control valve provided in the purge passage when the fuel vapor is rich. This prevents rich fuel vapor from being purged so that the vapor does not influence the air-fuel ratio control.
Recently, there has been a demand for preventing leakage of fuel vapor during refueling and while the vehicle is running. This demand is met by a system that recovers fuel vapor in the fuel tank during refueling without any leakage to the atmosphere. This system is known as a so-called ORVR (Onboard Refueling Vapor Recovery) system.
Specifically, this system has a breather passage provided, separate from the vapor passage, between a fuel tank and canister as described in the aforementioned Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-189426. In this breather passage is a diaphragm type differential pressure regulating valve, which opens according to the differential pressure between the pressure in the fuel tank and that in the fuel feeding pipe during refueling, which leads the fuel vapor in the fuel tank to the canister. At other times, the differential pressure regulating valve is closed to inhibit fuel vapor from flowing into the canister via the breather passage.
When a vehicle experiences strong vibration or accelerates or decelerates and the fuel in the fuel tank is greatly disturbed, however, the differential pressure regulating valve may open.
When there is a large amount of residual fuel in the fuel tank, much fuel is present in the fuel feeding pipe. In this case, the fuel may block the opening of a circulation line pipe that connects the upper portion of the fuel tank to the upper portion of the fuel feeding pipe to equalize their pressures. In this situation, the pressure passage that applies the pressure at the upper portion of the fuel feeding pipe to the differential pressure regulating valve is separated from the upper portion of the fuel tank by the liquid fuel.
When the surface of the fuel in the fuel feeding pipe falls according to the fuel surface in the fuel tank, the pressure at the upper portion of the fuel feeding pipe drops and the lower pressure is applied to the differential pressure regulating valve by the pressure passage. Meanwhile, the pressure in the fuel tank has been increased by disturbances in the fuel surface. This produces a differential pressure between the upper portion of the fuel feeding pipe and the inside of the fuel tank so that the differential pressure regulating valve may open, although refueling is not taking place.
During refueling, a lot of rich fuel vapor flows in the breather passage. Accordingly, rich fuel vapor remains in the breather passage after refueling. In some cases, this fuel vapor may condense so that fuel remains in liquid form.
As apparent from the above, it is possible that, although refueling is not occurring, the differential pressure regulating valve in the breather passage may open to discharge rich fuel or, in some cases, liquid fuel remaining in the breather passage to the canister.
As rich fuel is led to the canister, it is immediately drawn into the purge passage by the negative pressure of the intake air of the internal combustion engine. Most of the fuel therefore goes into the air-intake passage of the internal combustion engine from the purge passage without being adsorbed by the canister. This means that very rich fuel is supplied to the air-intake passage, changing the air-fuel ratio and significantly disturbing the air-fuel ratio control of the internal combustion engine. This is likely to adversely affect the stable combustion of the internal combustion engine.
To overcome this problem, the apparatus disclosed in the aforementioned Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-189426 has a second differential pressure regulating valve provided downstream of the first one to prevent the breather passage from being opened at times other than when refueling. This second differential pressure regulating valve opens based on the difference between the pressure in the breather passage and the atmospheric pressure.
However, the provision of two differential pressure regulating valves in the breather passage in the system of Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-189426 makes the evaporation fuel processing apparatus complex and increases the weight of the internal combustion engine.
The system disclosed in the aforementioned Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-144870 fails to respond to the opening of the breather passage and the residual fuel in the breather passage and the consequential effect on the air-fuel ratio control.
Accordingly, it is an object of the present invention to provide an air-fuel ratio variation suppressing apparatus which can suppress a change in air-fuel ratio in an internal combustion engine that is caused by the supply of rich fuel vapor through a breather passage at other times than the time of refueling, without complicating the breather passage or increasing the weight of the engine.
To achieve the above object, according to one aspect of this invention, an air-fuel ratio variation suppressing apparatus for an internal combustion engine, which is equipped with an evaporation fuel processing apparatus fuel processing mechanism for supplying fuel vapor in a fuel tank to a canister via a vapor passage, purging fuel in the canister to an air-intake passage of the internal combustion engine via a purge passage provided with a purge control valve when the internal combustion engine is operated, and supplying the fuel vapor in the fuel tank at a time of refueling to the canister via a breather passage provided with a pressure sensitive valve that opens in response to a variation in pressure in the fuel tank at a time of refueling, comprises determination means for determining an open state of the pressure sensitive valve; and suppression means for suppressing a variation in air-fuel ratio in accordance with a result of determination on the open state of the pressure sensitive valve made by the determination means.